Update: Now managed to reproduce this with 260.19.29 as well.. Seems to happen less frequently with this version, but that may just be a coincidence. Will test on a non-Nvidia system, to see if the problem might be with Flash..
Update 2: Tested on a non-Nvidia system with similar operating system environment, and Flash fullscreen works OK..

I was playing around now, but cannot reproduce the problem reliably.
But I've managed to lock X server while trying to reproduce the problem by
switching video from fullscreen to windowed mode and back.

Some xid appeared, however all screen was stuck on white.
I can only assume that corruption happens after those xids, but will try
to catch it again.